LiNbO.sub.3 and LiTaO.sub.3 integrated optic circuits (IOCs) are useful in fiber optic gyros (FOGs), photonic switching devices, and intensity/phase modulation systems. Their attributes include low optical loss, low voltage drive, high frequency bandwidth, and small size and weight.
The principal prior art method of fabricating LiNbO.sub.3 IOCs is by local diffusion of titanium (Ti) into a LiNbO.sub.3 or LiTaO.sub.3 substrate surface (i.e. Ti:LiNbO.sub.3 or Ti:LiTaO.sub.3). Through deposition and photolithographic techniques, a Ti pattern is defined and diffused to form optical waveguides on the substrate surface. The titanium diffuses to interstitial sites of the defined waveguide region, acting as an impurity dopant. This increases the ordinary and extraordinary refractive indices of the LiNbO.sub.3 or LiTaO.sub.3 substrate in that region, causing optical wave propagation of either polarization to be confined to the formed waveguide region by total internal reflection.
An alternative prior art method for fabricating optical waveguides in LiNbO.sub.3 and LiTaO.sub.3 is the proton exchange (PE) process. A masked LiNbO.sub.3 or LiTaO.sub.3 substrate is immersed in a molten bath of pure benzoic acid at a temperature below the decomposition point of the acid, causing lithium ions from the LiNbO.sub.3 surface region to be replaced by hydrogen ions from the bath. The process locally increases the extraordinary refractive index but decreases the ordinary refractive index, producing a polarizing waveguide. Light polarized along the extraordinary axis is guided but light polarized along the ordinary axis is not guided and leaks into the substrate.
In addition to supporting a single polarization the PE waveguides are easy to fabricate. PE diffusion occurs at 150-250.degree. C. for 5-30 minutes compared to 900-1100.degree. C. for 4-10 hours for the titanium-diffused guides. The PE waveguides fabricated in pure benzoic acid offer a larger refractive index change and, therefore, a tighter mode confinement than the Ti-diffused guides, and they are more immune to optical damage; however, they have four drawbacks.
First, they exhibit large propagation loss due to scattering, Second, the large index increase (.apprxeq.0.12) causes a large mismatch in numerical aperture with commercially available single mode fibers. Third, there are temporal instabilities in the refractive index distribution causing the propagation characteristics to vary with time. Fourth, and most important, there is evidence that the proton exchange process substantially degrades the electrooptic properties of the LiNbO.sub.3, making the PE guides less useful for active devices.
There have been prior art attempts to overcome these problems. One known improvement involves diluting the benzoic acid bath with a small percentage of lithium benzoate, to decrease the amount of Li.sup.+ being exchanged with H.sup.+. As reported by J. L. Jackel and C. E. Rice, Short And Long-term Stability in Proton Exchanged Lithium Niobate Waveguides, SPIE vol. 460, Processing of Guided Wave Optoelectronic Materials (1984), p. 43, for lithium benzoate concentrations greater than 3.5% the increase in refractive index is reduced by an order of magnitude. The range of index increase was 0.005 to 0.01 compared to a range of 0.10 to 0.12 for concentrations less than 3.5%. Metastable phases are not formed in LiNOb.sub.3 for lithium benzoate concentrations greater than 3.4% so that the waveguides are temporally stable.
A second and more practical method for making useful polarizing waveguides by the PE technique was demonstrated and reported by T. Findakly and B. Chen, Single-Mode Transmission Selective Integrated Optical Polarizers in LiNbO.sub.3, Electronics Letters, 1984, Vol. 20, pp. 128-129. The method known as the annealed-proton-exchange, performs two steps in which the waveguide is first formed by exchange in pure benzoic acid and then annealed at higher temperature for a proper duration. The annealing reduces the initial large index increase and expands the waveguide depth such that the waveguide numerical aperture can be properly matched to other single mode waveguides or commercial fibers. The same technique was also verified by J. J. Veselka and G. A. Bogert, Low Insertion Loss Channel Waveguides in LiNbO.sub.3 Fabricated by Proton Exchange, Electron. Lett. 23, p. 265 (1987).
Despite these improvements, the scientific community has not endorsed the use of proton exchange fabricated devices (fabricated by pure proton exchange without annealing) for active IO applications, such as switches and modulators. This is due to the poor temporal stabilities and low electrooptic efficiencies which have continued to be demonstrated by prior art devices. See for example: Nishihara et al. Optical Integrated Circuits, McGraw Hill Book Company, U.S.A., 1989, pp 161, and Theodor Tamir, Guided-Wave Optoelectronics, Springer-Verlag, West Germany, 1988, pps. 149-150.